1. Field of the Invention
This invention relates to a microstructure to be prepared by means of micromechanical technologies and it also relates to a method of forming a microstrucuture by using a sacrificial layer.
2. Related Background Art
In recent years, studies have been made to produce minute machines comprising small one or more than one mechanisms by means of micromechanical technologies. In particular, such a microstructure realized by applying technologies developed for forming semiconductor integrated circuits (semiconductor photolithography process) can be used to produce a number of minute mechanical components on a common substrate on a highly reproducible basis. Thus, minute components can be prepared for minute machines at reduced cost and such minute components provide an enhanced responsiveness if compared with conventional mechanical structures.
Three typical methods are popularly known for manufacturing a microstructure on a substrate. Firstly, there is a method for producing a microstructure such as a wobble micromotor of polysilicon film (M. Mehregany et al., "Operation of microfabricated harmonic and ordinary side-drive motors", Proceedings IEEE Micro Electro Mechanical Systems Workshop, 1990, pp. 1-8) or a linear microactuator (P. Cheung et al., "Modeling and position-detection of a polysilicon linear microactuator", Micromechanical Sensors, Actuators and Systems ASME 1991, DS C-Vol. 32, pp. 269-278), with which a sacrificial layer of silicon dioxide film and a thin silicon film of polysilicon, SOI (Si on Insulator) or SIMOX (Separation by ion implantation of oxygen) (B. Diem et al., "SOI (SIMOX) as a Substrate for Surface Micromachining of Single Crystalline Silicon and Actuators", The 7th International Conference on Solid-State Sensors and Actuators, transducers '93, Jun. 7-10, 1993, pp. 233-236) formed to produce a microstructure are patterned to show a desired profile and subsequently the silicon dioxide film is used as a sacrificial layer and removed with an aqueous solution of fluorine.
However, since this method involves the use of an aqueous solution of fluorine in order to etch out the silicon dioxide, a material that resists corrosion by fluoric acid has to be selected for the microstructure and hence such corrosive substances as aluminum cannot be used for the electrodes of the microstructure. Additionally, the polysilicon of the microstructure has to be controlled for its membrane stress in order to prevent it from warping. If an SOI substrate is used, the silicon dioxide supporting the microstructure can be etched back to consequently reduce the microstructure to show a beam-shaped profile and make it difficult to electrically connect the substrate and the structure when the silicon dioxide lying under the bulk Si thin film is removed.
Secondly, there is a known a method of forming a spatial light modulator comprising a micromirror of an aluminum (Al) thin film (L. J. Hornbeck, Japanese Patent Application Laid-Open No. 2-8812) by applying photoresist onto a substrate to form a sacrificial layer, forming thereon an Al thin film, patterning the Al film to a desired profile and thereafter removing the photoresist by dry etching using oxygen plasma to produce a microstructure comprising an Al thin film.
With this technique, it is possible to form a microstructure on a substrate selected from a variety of candidates because the sacrificial layer is made of photoresist, the use of which is not restricted by the surface coarseness of the substrate. Thus, the sacrificial layer can be removed by drying etching using the technology of reactive ion etching (RIE) and any possible sticking phenomenon that may appear between the microstructure and the substrate if a wet etching technique is used to remove the sacrificial layer can be successfully avoided. However, the process of forming the aluminum thin film for the structure has to be conducted at temperature that is low enough to prevent photoresist from being damaged by heat to impose rather rigorous restrictions on the selection of material for the microstructure. Additionally, since the microstructure is produced by means of a thin film forming technique such as vacuum deposition or sputtering, the membrane stress of the thin film has to be so controlled as to prevent the microstructure from being warped by the membrane stress.
Last but not least, there is known a method of forming the pattern of a microstructure on a bulk Si substrate, coupling part of the pattern to the glass substrate by anode coupling and etching the coupled Si substrate from the rear surface in such a way that only the microstructure is left on the glass substrate. A linear actuator made of bulk Si thin film formed from an Si substrate (Y. Gianchandani et al., "Micro-Size High Aspect Ratio Bulk Silicon Micromechanical Devices", Proceedings IEEE Micro Electro Mechanical Systems Workshop, 1992, pp. 208-213) and a cantilever for an AFM (Atomic Force Microscope) made of silicon nitride film (T. A. Albrecht et al., U.S. Pat. No. 5,221,415) can be prepared with this technique.
Since this method does not involve the use of a sacrificial layer, a microstructure can be made of a material that may be corroded by fluoric acid. However, since it has to be coupled with glass by anode coupling, candidate materials are limited to electroconductive silicon that can be easily oxidized, metals such as Si, Al, Ti and Ni and silicon nitride or oxide that can be anode-coupled only in the form of a thin film formed on a Si substrate. Additionally, since the process of anode coupling has to be conducted at temperature higher than 300.degree. C., the glass to be coupled with a microstructure is required to show a thermal expansion coefficient substantially equal to that of the Si substrate. Thus, glass that can be used with this method is limited to Pyrex glass (e.g., #7740 Corning: tradename). Additionally, since a void has to be formed in advance on the coupling surface, electrodes cannot be arranged on the microstructure after the coupling operation. Moreover, since the substrate has to be made of glass containing mobile ions, circuits cannot be formed on the substrate to a high degree of integration. Finally, since the surface coarseness of the glass and the electroconductive material has to be held to less than 500 angstroms for coupling the glass and the electroconductive material by anode coupling, they cannot be coupled on stepped wires.